1. Field of the Invention:
The present invention relates to an apparatus and method for photographing the fundus of an eye. More particularly, the present invention relates to an apparatus and corresponding method which obtains images of a plurality of different portions of the fundus of the eye, and arranges those images to create a mosaic image of a section of the fundus of the eye comprising the plurality of different portions.
2. Description of the Related Art
In industrial countries, the most common eye diseases, aside from cataract, are diabetic retinopathy, glaucoma and age macular degeneration. Although these diseases can lead to severe losses in vision, if treated at the appropriate stage, the risk of vision loss is reduced significantly.
In order to detect the onset of treatable disease, persons at risk should be examined by an eye care specialist on a regular schedule. Unfortunately, only a portion of these persons is examined routinely. For example, on average, half of all diabetic patients do not visit an ophthalmologist as recommended. This lack of monitoring, not related to the availability of care, leads to an unnecessary elevated incidence of vision loss accompanied by low quality of life, high cost of health management, and loss of productivity of persons affected and their custodians. There is thus an effort to find available means to screen for common eye diseases.
Since those at risk for such eye diseases generally make regular visits to primary care physicians, it is advantageous to perform at the physicians' offices. However, at present, there are no adequate devices to perform the screening. The devices must be easy to use by office staff, rapid, sensitive, as accurate as possible and, most importantly, affordable.
Photography has been used for aiding in the diagnosis of common eye disorders, and is often considered superior to ophthalmic examinations for the detection of a number of diseases and diagnosis of numerous eye disorders such as diabetic retinopathy and glaucoma. For diseases like diabetic retinopathy, photographs allow the eye care specialist to detect the presence of pathologies such as abnormal blood vessels, deposits of components, such as lipids that have leaked from the vessels, and edema. For detecting glaucoma, the photographs are useful for examining the optic disc and its vicinity for loss of nerve fibers.
In addition, diagnostic methods are used to assess vision. Typically, losses are detected by psychophysical tests which evaluate the response of the subject to visual stimuli.
Unfortunately, these proven methods at present are only available to eye care specialists. A need therefore exists for a system which enables these tests to be performed in a manner that fits the practical needs of a plurality of screening environments, such as primary care physicians offices, optometrists offices, large places of employment place and mobile units.
In order to screen for common eye diseases efficiently, the imaging system must provide images with a relatively large field of view (e.g. about 50.degree.) measured as the conical angle originating at the pupil and extending towards the area on the retina being imaged. Such a field of view is sufficient for reliable detection of common diseases such as diabetic retinopathy, glaucoma and age related macular degeneration. The imaging system should also provide a resolution of 60 pixels or more per degree. Conventional fundus photographic images are currently acquired on film with a resolution adequate for diagnosis as long as the field of view is 30.degree. or less. The resolution of conventional photographs on film is about 60 pixels per degree. Therefore, a total of approximately 1800 pixels (measured diagonally) is achieved (i.e., 30.degree..times.60 pixels per degree=1800 pixels). For the desired field of 50.degree., a total of 3000 pixels measured diagonally would be achieved.
Stereo imaging with a constant stereo angle is also desirable in such an imaging system. The detection of macular edema, a common cause of vision loss, is routinely based on the examination of stereo pairs of fundus images to detect retinal thickening. The stereo angle is the angle between the two imaging paths of the two images. By viewing these images, each with a different eye, the examiner obtains a sensation of depth. This stereo effect is enhanced by increasing the stereo angle. Therefore, to obtain a stereo sensation that can be used to compare images, the stereo angle should be constant.
The system should also allow for efficient operation by non-ophthalmic staff. The most efficient method to screen for common eye diseases can be achieved by examining the eye during routine visits to primary care physicians, as opposed to requiring visits to an ophthalmic specialist. Thus, the camera should be specifically designed for operation by nonophthalmic staff. The system should further provide cost efficient screening. For screening to be incorporated in routine health care delivery, the cost needs to be modest and commensurate with the financial and medical benefit.
Additionally, the system should be capable of performing imaging without pharmacological pupil dilation. Conventional fundus photography requires pharmacological pupil dilation by use of a topical instillation of a drug that dilates the pupil to prevent it from constricting upon exposure to the light necessary for photography. Imaging without the installation of a drug makes the process easier and quicker. This requirement is not crucial because pharmacological pupil dilation is a common procedure used for thorough eye exams.
Conventional fundus cameras yield the desired resolution for a field of view of only up to about 30.degree., which is much smaller than the preferred 50.degree. field of view. To cover the desired larger area, the photographer is required to manually aim the camera to multiple adjacent regions and obtain multiple photographs. Stereo images are acquired by obtaining one photograph through the right side of the pupil, then manually moving the camera to take a second image through the left side of the pupil. This procedure yields images with an unknown stereo base and thus an unknown and varying perception of thickness and depth. In order to yield images adequate for diagnostic purposes, conventional fundus imaging and stereo imaging necessitate operation by a trained ophthalmic photographer. Pharmacological dilation is also typically needed. Finally, the cost and the inconvenience of film make these cameras inadequate for screening.
Digital cameras that yield images with 2000 pixels in diameter have been optically coupled to conventional fundus cameras and have yielded 30.degree. field images with adequate resolution. As the digital cameras record the image electronically and store them in memory, there is no need for film. However, such systems are inadequate because they share the same drawbacks mentioned for the conventional camera to which they are coupled. In addition, the elevated price of these digital cameras adds to the cost of the screening, rendering it unattractive in locations with a small volume of target individuals.
Some fundus cameras provide images or markers to assist the operator in the alignment of the pupil. One known camera provides auto focusing. Other cameras have been designed to obtain images without pharmacological dilation. Still other cameras have been specifically designed to obtain simultaneous stereo pairs, but their resolution is reduced because each frame on the film is shared by the two images that form the stereo pair. None of these cameras provide a large enough photographic field at the required resolution.
Furthermore, as mentioned above, psychophysical tests, namely, tests performed to assess the mental perceptions of physical stimuli, are important in ophthalmology. Most of these tests are intended to detect pathology in the retina or the neural pathway. Visual acuity and visual field tests (perimetry) are an example. Visual acuity assesses central vision, namely, the ability of the subject to perceive small objects, and perimetry testing is aimed at detecting losses of vision mostly in the more peripheral region of the fundus. However, when faced with a response below that of normal subjects, the vision care specialist may have difficulty in determining whether the reduced response is due to optical obstacles in the media or caused by retinal and neuroretinal abnormalities. The management of the patients depends on this differential diagnosis. For example, a reduced visual field response can be due to insufficient dilation of the pupil or opacities in the lens. A poor visual acuity could be caused by opacities and optical aberrations. Finally, in tests such as perimetry, it is difficult to assess the location on the fundus that is responsible for the abnormal response.
Accordingly, a continuing need exists for a system which is capable of obtaining a photographic image of the fundus with the desired resolution and field of view, as well as a system which is capable of measuring visual acuity and visual field.